A judge took the unprecedented step this week of publicising the apparent abduction of four children involved in a custody battle between their Welsh mother and Spanish father. After being ordered to return her children to their father, Jennifer Jones allegedly absconded from her Carmarthenshire home with Jessica, 14, Tomas, 12, Eva, nine and David, eight. Airports and ports were put on standby. They were found 24 hours later, when the mother was arrested with her new partner and the children were placed in care before being taken back to Spain.

The precise circumstances surrounding this case are not clear. However, in the past year, British consular assistance for help in child residence cases involving a partner from another country has been sought 17 to 29 times each month. There has been no rise in the total number of cases, but experts say there is one area where cases abroad are increasing, albeit one in which no statistics are collected and no records kept.

John Mellor, service manager at Children and Family Court Advisory and Support Service (Cafcass), who specialises in cases of child abduction and international relocation, says he is seeing an increasing number of cases where a British couple split up after moving abroad with their British children.

When one parent – usually the mother – wants to return to the UK with the children, Mellor says he or she finds out too late that the children are under a foreign jurisdiction and the British courts have no say over their future. "This situation is becoming more common given global patterns of migration combined with the economic situation, which is leading to expat couples finding their jobs and businesses abroad collapsing," he says.

Having a custody case heard in a foreign court, warns Mellor, can be highly detrimental to the mother's case. "Unlike in the UK, where complex international custody cases are transferred to a higher court, in many foreign countries, a family court is usually quite a low-level judge in a little, local court which will never have dealt with one of these cases before and probably never will again."

Decisions, he adds, tend to be made "on a pretty hit-and-miss basis. There's little expertise, knowledge and understanding informing the decisions of those judges. Whichever parent puts the most compelling argument or plays to that judge's prejudicial assumptions will win. The judges often have default positions, like 'mothers who go away are not good mothers', or that 'a mother's place is in the home, no matter what'."

Anne-Marie Hutchinson, who won the international family lawyer of the year award and an OBE for her services to international child abduction, has dealt with at least six cases this year of British expat couples who found themselves fighting their custody battles in a foreign court.

"It happens in the UK too," she added. "There are lots of eastern European women who came over to Britain with their families when the economy was strong. They want to go back home to their friends and families but their husbands won't let them take the children."

It is the mothers, she says, who tend to come off worse when fighting for custody of their children in a foreign court. "Women often can't get interim leave to bring their children home but also can't afford to stay abroad. Their former partners successfully spin them as having abandoned their children. But if they stay, they're penniless and thus have no prospect of persuading the court they can look after their own children."

For the past two years, Mary has been battling with a Portuguese court for permission to bring her two young children back home with her to England.

Idyllic

Life was idyllic when Mary and Dan first moved to Portugal with their two young children. The British-born family bought land in an isolated part of the country and set about building their new home, living in a bus they had driven over from England and eating food they had grown themselves.

The relationship unravelled in 2010, four years later. She wanted to come home but Dan refused and insisted the children, now seven and five, stay with him. To her surprise, Mary discovered the case had to be heard in a local, Portuguese court: by moving abroad, her children's country of habitual residence had shifted.

"International law makes it almost impossible for mothers to get a foreign court's permission to take their children back home," says Mary, who has asked for names to be changed for fear that talking to the media will harm her case. "If I stay in the UK during those months, I could lose the court case by default because Dan will claim the children are, by then, happy in Portugal and settled without me," she says. "But if I give up my life in England to go back out to Portugal, I will automatically lose my case because I will no longer have a house or a job back home to offer them."

Mary is now studying full-time for a postgraduate certificate in education and has her own house in the north of England. She insists it is impossible for her to live in Portugal. "I can't make a life there," she says. "The farm is in the middle of nowhere. The closest two schools – the only places I could find work that would pay well enough to enable me to survive on my own – are a three-hour drive away, which means I would only be able to see the children one day a week. Added to which, jobs don't come up at either school very often so it's highly unlikely that I could even find enough work to be able to afford to stay in the country.

Isolated

"I would have no savings, house or social benefit system to fall back on," she adds. "I would be completely isolated: I don't speak Portuguese. I have few friends and no family out there."

In contrast, says Mary, Dan could easily find employment in the British city where she now lives: the couple lived and worked there before moving together to Portugal. "The children need their mummy," she says. "It is not a choice between whether the children should live either with Mum or Dad, it's either 'Dad alone in Portugal' or 'Mum with family and optional dad in England'. Dan has the choice whether he lives in England. I cannot live in Portugal."

Alison Shalaby is acting director of Reunite, a UK charity dealing with the movement of children across international borders. She says her charity, part-funded by the Ministry of Justice and the Foreign Office, is handling an increasing number of similar cases from countries including Australia, New Zealand and Hong Kong. They have had 16 this year already. Another 19 women have rung for advice.

"It's not about nationality or passports: by moving to a foreign country with a settled purpose, you are changing your children's country of habitual residence," she said. "That can happen after just a couple of months.

"If parents can't decide where their children should live, the best-placed country to make that decision for them is the country of their children's residence," she said. "If you choose to live in another country, you have to abide by the laws of that country."

In the meantime, Mary says it is only because her new partner has paid for her flights and accommodation in Portugal over the past year that she has not already lost her case. "If I had not met my new partner, I would not have been able to afford to see my children in Portugal and build a life for them in the UK at the same time. Apart from the emotional impact on my children and myself, that would have been devastating for my case.

"The law is wrong. I hate to say it, but I can see this ending up with me losing my children," she admits. "I can't bear to think what my life – or theirs – will be like then. It's beyond imagining."